Punnett Squares

I was re-reading Harry Potter and the Methods of Rationality and came across the part where Harry tries to explain genetics—

*cough* If you haven’t read this fanfiction, it’s pretty darn awesome. Harry Potter meets Science.

—anyway, I won’t spoil it, but they were talking about a certain genetic trait. The phenotype is either +W or -W, and they were trying to figure out what the genotype was. Harry explains the monohybrid cross you probably learned about in high school Bio, where there are two alleles, W and w. To find out how the genotype and phenotype relate, they counted how many children were +W when the parents were both Ww.

At this point you ought to be thinking “Punnett square”, and if not you should click on that link and look at the table. Because about a quarter of the children were +W, Harry concludes that the W trait follows a recessive inheritance pattern—you need two copies of the W allele to have a +W phenotype.

The problem? While it was established that two -W parents can have a +W child, it’s also implied that two +W parents can have a -W child. That’s just not possible with a recessive inheritance and a single gene.

Looking back, it’s possible that they meant that one +W parent and one -W parent, in which case -W is certainly possible. But it seems more likely that there are at least two genes which affect +W, which would allow both kinds of unusual children.

But more importantly, I realized that I was thinking about genetics from a fanfiction based on young adult fantasy, and decided to let it go.